Prepared in cooperation with the National Park Service Acid Rain in Shenandoah National Park, Virginia Visitors to Shenandoah National Park (SNP) enjoy the animal and plant The pH scale is a measure of how acidic (low pH) or alkaline life and the scenery but may not real- (high pH) a solution is. Rainwater is considered normal at 5.6 pH ize how vulnerable these features are to units. Shenandoah National Park rain typically is 10 times more various threats, such as invasion of exotic acidic than normal rain. plants and insects, improper use of park resources by humans, and air and water pollution. The National Park Service rain (currently about 4.6 pH units) falling mic, which means that each whole-num- strives to protect natural resources from onto an environment that has little inher- ber change indicates a 10-fold change in such threats to ensure that the resources ent ability to neutralize the acidic input acidity or alkalinity. For example, a pH of will be available for enjoyment now and and decades of exposure to acid rain have 4 is 10 times more acidic than a pH of 5. in the future. Because SNP has limited resulted in a fragile environment. When Rainwater is considered normal at 5.6 pH influence over the air pollution that the effects of acid rain are combined units; therefore, rain with a pH of 4.6, envelops the region, acidic deposition— with stressors, such as forest defoliation which typically occurs in SNP, is about commonly known as acid rain—is one of caused by the gypsy moth or conifer- 10 times more acidic than normal rain. the more challenging threats facing park ous tree demise caused by the hemlock Acid-neutralizing capacity (ANC)— managers. With the help of U.S. Geo- woolly adelgid, significant environmental the ability of a solution to neutralize logical Survey (USGS) scientists, park problems can develop, and park resources acidic inputs—is a quantitative measure managers can understand how acid rain become threatened. associated with pH (fig. 1). In general, interacts with ground- and surface-water water with a high ANC (for example, resources, which enables them to explain 500 units1) also has a high pH and is why reductions in air pollution can help good at neutralizing acidic inputs. In preserve park resources. Such under- Understanding pH and contrast, water with a low ANC (for standing also provides essential insight Acid-Neutralizing example, 25 units) has a low pH and is into ecosystem processes, as managers a poor neutralizer. Stream animals and strive to unravel and resolve other envi- Capacity plants (the biota) can be sensitive to and ronmental problems that are interrelated negatively affected by changes in ANC, to acid rain. The severity of acid rain and acidic particularly when it decreases to near Even though reductions in emissions streamwater is measured by the pH zero units, which can cause the biota to of acid-forming compounds mandated by scale—a quantitative measure of how suffer “acid shock.” Research on fish the Clean Air Act and Amendments have acidic (low pH) or alkaline (high pH) a indicates that many species are sensitive occurred since 1990, acid rain contin- solution is (fig. 1). A neutral solution, at ANC values less than 50 units. ues to fall in the eastern United States, which is neither acidic nor alkaline, has including SNP. The combination of acid a pH of 7 units. The pH scale is logarith- 1ANC concentration typically is expressed in microequivalents per liter. “Unit” is used here for simplifying purposes. Acid rain Fish begin Normal Baking Battery acid Vinegar to die rain soda Ammonia Lye 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Figure 1. Relations Acidic Alkaline of common solutions Neutral and acid-neutralizing capacity to the pH Low acid-neutralizing capacity Intermediate acid-neutralizing capacity High acid-neutralizing capacity scale. U.S. Department of the Interior Printed on recycled paper Fact Sheet 2007–3057 U.S. Geological Survey July 2007 Shenandoah National Park Streams Acid Rain Facts Shenandoah National Park streams start near the top of a mountain, usually as a spring or seep on a hillface. As the • Acid rain is caused by combustion of fossil fuels (gas, oil, coal) and to a much water flows downhill, the stream increases lesser extent by natural processes (volcanic emissions, forest fires). in size as more tributaries and ground • Combustion emits sulfur dioxide and nitrogen oxides (the two major contributors water contribute to the streamflow. Such to acid rain) into the atmosphere; these oxides can travel hundreds of miles from streams are called “headwater streams.” the source of emission. Because these streams begin with little water, contributions of acid rain can make • Sulfur dioxide and nitrogen oxides convert to sulfuric and nitric acids in the the streamwater more acidic. During atmosphere; the acids are deposited to the earth by wet deposition (rain, snow, storms, acid rain quickly reaches head- fog) and dry deposition (dry particles, gases), collectively referred to as “acidic water streams and can decrease the ANC deposition.” and pH of the stream temporarily. As the pulse of acidic water flows downstream, • Some of the largest emissions in the United States originate in the Ohio River the streamwater recovers to the pre-storm Valley and are carried downwind through the atmosphere toward Shenandoah ANC and pH values. National Park. Although the acidity of rain is spa- • The Clean Air Act, designed to reduce emissions, was passed in 1970, amended tially uniform across SNP, streamwater in 1990, and amended again in 1995. ANC and pH values vary substantially across the park. These differences are • In Shenandoah National Park, the annual average pH values of precipitation related to the underlying geology (bedrock for the following periods were: 4.53 for 1981–1990; 4.59 for 1991–1995; type) and topography (steepness of the 4.62 for 1996–2005. These pH values suggest incremental improvements in slope) of the watersheds. The three main response to the Clean Air Act amendments. bedrock types in SNP are basaltic, granitic, and siliciclastic. Basaltic rocks are the • Rain tends to be most acidic during the summer; longer periods between storms best of the three bedrock types at neutral- allow acids to build up in the atmosphere before being “rained out.” izing acidic inputs, siliciclastic rocks are • Individual pH values of rain well below 4.0 have been measured in Shenandoah poor, and granitic rocks are intermediate. National Park. Each of the three bedrock types underlies approximately one-third of SNP. Shenandoah National Park has 231 headwater streams in which ANC and pH decrease to various degrees, depending on the geology and topography. In general, Streams start as the steeper the slope, the more quickly acid Streamwater Streamwater seeps on a hillface rain reaches the stream with less chance of pH = 6.1 pH = 4.5 ANC = 150 More desirable environment being neutralized along ANC = 5 for fish and other aquatic biota the way. Therefore, a Poor environment for a small stream near the diverse population of fish Streamwater crest of a mountain, pH = 7.2 and other aquatic biota ANC = 250 on a steep slope, and Soil underlain by siliciclas- tic bedrock, is likely Basaltic Bedrock to have some of the Streamwater most acidic water in pH = 4.6 Siliciclastic Bedrock ANC = 15 SNP (fig. 2). Such streamwater does not support a diverse popu- lation of aquatic biota. Conversely, a large stream with a gentle slope and underlain by basaltic bed- Figure 2. Two stream systems with different characteristics that result in differences in water quality. rock will have near-neutral Whether or not the aquatic biota will be diversified is strongly controlled by the geology and topography. water and, consequently, a diverse population of aquatic biota (fig. 2). Front Royal Entrance Station Front Royal Washington, D.C. Shenandoah Dickey Ridge Visitor Center Waynesboro National Park CHESAPEAKE Richmond Mathews Arm BAY 78o15' VIRGINIA ATLANTIC OCEAN 38o45' Elkwallow Shenandoah National Park Facts Skyline Drive • Consists of 308 square miles (nearly Thornton Gap Entrance Station 200,000 acres) • Straddles the crest of the Blue Ridge Mountains Panorama • Stretches from Front Royal in the north to Rockfish Gap in the south, with Skyline Drive traversing the entire length Skyland • Touches eight counties (Albemarle, Augusta, Byrd Visitor Center Greene, Madison, Page, Rappahannock, 78o30' Rockingham, and Warren) 38o30' • Has 231 headwater streams Big Meadows • Contains three main types of bedrock Skyline Drive (basaltic, granitic, and siliciclastic) Lewis Mountain • Has land-surface elevations that range from about 600 to 4,051 feet Swift Run Gap Entrance Station 78o45' EXPLANATION MINIMUM ANC VALUES 38o15' Skyline Drive Less than or equal to 50 units (poor ability to neutralize acidity) Loft Mountain Greater than 50 units (better ability to neutralize acidity) Shenandoah National Park boundary Watershed boundary 0 5 10 MILES 0 5 10 KILOMETERS Rockfish Gap Entrance Station Figure 3. Predictions of Shenandoah National Park watersheds that are likely to have a minimum average acid-neutralizing capacity (ANC) less than or equal to 50 units (red) or greater than 50 units (blue) for 6 consecutive hours about once every 4 years. The watersheds shown in red have the greatest likelihood of losing native aquatic species because of extended periods of low ANC. Models and Predictions These results are of concern because Scientists studied streamwater data from five SNP streams and developed a they indicate a high probability for loss specific mathematical model for each of of native aquatic species in the park. the five watersheds based on mathemati- cal relations among ANC decreases in each stream and the geologic and topo- graphic characteristics of the watershed One-half of the SNP watersheds are For additional details about the (Rice and others, 2005).